Structural Aspects of Protein Accumulation in Developing Pea Cotyledons. I. Qualitative and Quantitative Changes in Parenchyma Cell Vacuoles

Abstract

Structural changes in pea cotyledons during development were studied using light and electron microscopy. Changes in the vacuolar system and cytoplasm of cotyledon parenchyma cells, during the period of storage protein deposition, are reported. Eight days after flowering, the parenchyma cells each contain one or two large vacuoles that are replaced by progressively smaller vacuoles during the next 10 days of development. Stainable material that can be histochemically identified as protein appears on the inner surface of the vacuole tonoplast 8 days after flowering. These vacuoles become smaller and more frequent during development and the amount of proteinaceous material within each vacuole increases until, at days 16-20 after flowering, they become densely packed with protein and are described as protein bodies. At day 8, the vacuole(s) have an average diameter of 39 µm, an average volume of 41 000 µm³ , representing 75 % of the cell volume, and a surface area of 5500 µm². By day 20, the average protein body diameter has fallen to 1 µm. There are, however, approx. 175 000 such protein bodies per cell, occupying 91 500 µm³ or approx. 20 % of the cell volume, and whose total surface area is 550 000 µm². The surface to volume ratlo of the vacuole/protein bodies Increases 55 times between days 8 and 20. Apart from this increase in surface area available for possible entry of protein, no mechanism for such entry can be suggested from our nlicrographs.